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Dm. W, 194:6.i R. 1r. PALMER 2,432,782 WET BULB THERMOMETER AND THERMOSTAT Filed May 26, 1944 2 Sheets-Sheet l u u Dec. 17, i946. R. T. PALMER l ¿M2782 WET BULB THERMOMETER AND THERMOSTAT Filed May 26, 1944 2 Sheets-Sheet 2 kmq T13-"50NTRÜLÜ Es@i Patented Dec. 17, 1946 2,412,782 -UNITED srA'rEs vPii'nzNT oFFlc‘E v 2,412,782 WET BULB THERMOMETER AND THERMOSTAT Robert T. Palmer, Sharon, Mass. Appllcûtlon May 26, 1944, Serial N0. 537,507 12 Claims. (Cl. 23e-_44) 2 This invention relates to instruments respond ing to wet bulb temperature changes such as are commonly known as wet bulb thermometers and wet bulb thermostats. _ This application is a continuation-in-»part of by copending application, Serial No. 428,312, filed January 26, 1942. Wet bulb thermostats are desirable for such duties as regulating air cooling systems in accord ance with latent heat changes. The usual wet bulb thermostat is a dry bulb thermometer utilizing a water soaked wick around its bulb for obtaining a wet bulb depression through evapora tive cooling. Such thermostats have not been ' extensively used due to the diiliculties of supplying the correct amount of water to the wicks and ci’ keeping the Wicks clean in theAA out-of-way locations such as in ducts, where control thermo stats are usually located. ` , For example, in one embodiment of this inven tion, the humidity of the air determines the power absorption of a hygroscopic dielectric between turc condenser electrodes in an electrostatic iield, While the dry bulb temperature determines in dependently, the strength of the field between the electrodes. A humidity increase causes an incrase in power absorption due to the increased absorption of moisture by the hygroseopic dielec tric. A dry bulb temperature increase causes a bi-metallic element to increase the spacing be tween the electrodes, thus decreasing the capacity and the power absorption. The two changes are so correlated that a meter or a control in the oscil lator circuit indicates or responds, respectively, to the prevailing wet bulb temperature. In another embodiment of the inventiony a humidity increase in the air may cause the fre quency oi an alternating electric current to in This invention provides wet bulb thermostats and thermometers which do not require water 20 crease, while an increase in the dry bulb tempera supply and which are electronic in action. This invention depends in principle, upon the fact that as shown by standard psychrometric charts, relative humidities, dry bulb temperatures ture of the air may cause the frequency to de crease. The frequency changes are so correlated that the resultant of the changes indicates the wet Vbulb temperature of the air. and wet bulb temperatures are so related that 25 A humidity increase in the air to which a device embodying this invention is exposed, may also the relations between any two of these three cause an impedance change in an electric circuit variables enable the third to be determined. For while a dry bulb temperature increase may cause example, suppose it is desired that a wet bulb thermostat actuate a control at 70° F. wet bulb 30 an impedance change which is opposite in eiîect to that caused by the humidity increase. For temperature. For this wet bulb temperature example,v a hygroscopic resistor connected in an Relative Dry bulb Percent Degrees B0 75 70 60 50 40 77 80 84 88 humidity temperature electric circuit, decreases in resistance with in crease in humidity while the resistance of another resistor in series therewith, is increased with in crease in the dry bulb temperature. Objects of the invention are to measure wet bulb temperatures and to actuate controls in response to wet bulb temperatures, by electronic action. 40 The invention will now be described with refer 'I'his invention causes the varying factors of dry ence to the drawings, of which: bulb temperature and relative humidity elec Fis’. 1 is a diagrammatic view of one embodiment tronically to determine and to indicate wet bulb of this invention utilizing hygroscopic, book type temperatures, and to actuate controls at pre electrodes, the capacity between the electrodes be determined wet bulb temperatures. 45 ing varied by a bi-metallic element responsive to A moisture increase in the air to which a device embodying this invention is exposed, may cause a dry bulb temperature changes. frequency or intensity change in electric current flow while an increase in the dry bulb tempera condenser having plates coated with hygroscopic change in current flow. The said current ilows type condenser of Fig. 1; Fig. 2 is a diagrammatic view illustrating a ture may cause an opposite frequency or intensity 50 material and having plates rotated by a bi-metallic spring, and which may be substituted for the boola are so evaluated with respect to the values shown by a psychrometric chart that the resultant fre quency or intensity of the current is a measure of the wet bulb temperature of the air. A Fig. 3 is a diagrammatic view of a condenser which may be substituted for the book' type con denser of Fig. 1, having two sets of rotary plates, 55 one set being rotated by a coiled hygroscopic ele 2,412,782 ' 4 3 ment affected by humidity changes and the other set being rotated by a coiled bi-metallic element affected by dry bulb temperature changes. Fig. 4 is a sectional view along the lines 4-4 of Fig. 3; ' Fig. 5 is a sectional view along the lines 5-5 of Figs. 2, 3 and 6; Fig. 6 is a diagrammatic view of another em bodiment of this invention utilizing an induct ance instead of a condenser for responding to' humidity changes, the inductance having rela tively fiat wires coated with hygroscopic material affected by humidity changes, the inductance be ing connected by a variometer to the oscillator, electrodes to the high frequency oscillator I3, an oscillating high frequency electrostatic iield being set up between the plates III and I I. The similar free electrode I4 is placed between the electrodes I0 and I I and acts to increase the capacity of the condenser formed by the electrodes II) and Il. The inner surfacesof the electrodes IIJ and I I and both surfaces of the electrode I4 may be coated with a hygroscopic medium such, for ex ample, as cellulose acetate dissolved in acetone and applied as a lacquer to the surfaces. The i moisture absorbed by the coatings varies as the relative humidity of the air passing between the electrodes changes. The moisture absorption the' rotor of the variometer being rotated by a 15 changes the condition of the dielectric between the electrodes and thereby changes the capacity coiled bì-metallic spring aiïected by dry bulb> tem and reactance of the condenser and changes the perature changes; power absorbed by the hygroscopic dielectric from Fig. '7 is a diagrammatic view of one oscillator the electrostatic ñeld. circuit which may be employed in the practice of The moisture absorption of the hygroscopic 20 this invention; l dielectric varies conformably with changes in the Fig. 8 is a diagrammatic view of anotherV oscil relative humidity of the air. The spacing be lator circuit which may be employed in the prac tween the electrodes I0 and` Il varies conform tice of this invention; ably with changes in the dry bulb temperature. Fig. 9 is a diagrammatic view cfa third form > of oscillator circuit which may be used in the 25 The constants of the apparatus are so chosen as practice of this invention; Fig. 10 is a diagrammatic view of another em bodiment of this invention in which a hygro scopic resistor is connected in series with a vari able resistor in the grid circuit of a thermionic electron tube, the resistance of the variable re sistor being adjusted by a dry bulb thermometer, the two resistors being in shunt with a D. C. volt- . age source; Fig. l1 is a diagrammatic view or' another em bodiment of this invention, similar to that of Fig. 10 except that the voltage source is con nected in series with the variable resistors; Fig. l2 is a diagrammatic view of another ema bodiment of this invention, similar to that of Fig. 10 except that the electron tube is of the cold cathode type, and the voltage source need .not be direct current. by observation of the effects through comparison with calibrated meters, that the resultant effect upon the oscillator I3 varies’conformably with changes in the wet bulb temperature of the air. The meter I5 connected in the oscillator cir cuit as will be described, serves to indicate the wet bulb temperature of the air, and the relay I6 connected in the oscillator circuit, serves to cause the device to act as a wet bulb thermostat for actuating controls of humidity changing devices when the oscillator current reduces a predeter mined value. instead of coating the surfaces of the electrodes with a hygroscopic layer, a dielectric such as reverted cellulose may be placed between the elec trodes as disclosed in the Allen Patent No. 1,708,073. bodiment of this invention, similar to that of Fig. 9 except that instead of the thermostatic element The oscillator i3 may be of the type disclosed in said Allen patent, or may be of the types dis-> closed in the Stevens et al. Patent No. 2,231,035, or may be of the type disclosed in the Terman varying a capacity, it varies a resistor in series Patent No. 1,976,904. Suitable circuits whichv with the hygroscopic resistor; lFigli is a diagrammatic view of another em» may be used will be described in the foregoing in connection with Figs. 7-9. Fig. 2 discloses a modiiied form oil condenser which may be used. It is a conventional tuning condenser having stator plates Il and rotor plates i3. rThe rotor plates i8 are rotated by a coiled bi-metallic member I2 such as shown by Fig. 5 so that increases in dry bulb temperature Fig, i3 is a diagrammatic view of another em bodiment of this invention using a relaxation type, resistance tuned oscillator; Fig. l5 is a diagrammatic view of another em bodiment of this invention similar to that of Fig. l1, except that a resistor, the value of which varies with temperature, is connected in series in the grid circuit of they control tube with a resis tor, the value of which is varied with humidity, and - ' Fig. 16 is a diagrammatic view of another em bodiment of this invention utilizing an ohmmeter and a rectiñer bridge circuit energized by alter» nating current. - Referring now to Fig. l, the spaced metallic plates 10 and l1 form a book type condenser, the plate Ill being adapted and arranged to be rotated by the coiled by-metallic member I2 so that the spacing between the plates increases with in creases in the dry bulb temperatures, and de creases with decreases in dry bulb temperatures. 'The capacity of the condenser thus decreases while its reactance increases, with increases in dry bulb temperatures, and its capacity increases while its reactance decreases, with decreases in dry bulb temperatures. The plates l0 and Il are connected as opposed cause reductions in capacity as described in con nection with Fig. 1. The member l2 is mounted onv the shaft I9 on which the plates i8 are mount ed, with its inner end in a slot in the shaft and its outer end held by the bracket 20 which is at tached to the case 2l. The plates I‘i and i8 are coated with hygroscopic material as described in connection with Fig. l, the overall eiïect being as in Fig. 1, of the hygroscopic absorption, and of dry bulb capacity variation, on an oscillator con nected'to the condenser of Fig. 2, to cause a re sponse to wet bulb temperature. Fig. 3 shows another form of condenser which may be used. Instead of the condenser plates being coated with hygroscopic material, it has two sets of rotary plates, one set being rotated by a coiled hygroscopic tape 22 in the case 23 while the other set is rotated by a coiled bi-me tallic member as described in connection with Fig. 2. The humidity changes cause the tape 22 asians: to rotate the plates more in mesh with the stator plates Ill to increase the capacity of the con denser, while the spring I2 rotates the other set be calibrated to read wet bulb temperatures, and o1' rotor plates I8 to decrease the capacity of the condenser. The resultant capacity varies con ture occurs. The oscillator'circuit of Fig. 9 includes a re~ the relay I8 may be adjusted to close a control circuit when a predetermined wet bulb tempera iormably with wet bulb temperature changes. sistance tuned oscillator of the type disclosed in The tape 22 may be a strip of cellulose applied pages S33-,344 of the June, 1941, issue of Pro to a strip of lshim brass as disclosed in the ceedings of the Instituteof Radio Engineers. The Rolleison Patent No. 2,093,767, and fastened to tuning condenser 38 has its capacity varied with the rotary shatt of its associated rotor plates as 10 changes in dry bulb temperature as disclosed in described above in connection with the support connection with Fig. 2 so as to decrease the capac of the Ímember I2 upon the shaft I9. » ity and increase the reactance upon increase in The rotor plates 2l could also be rotated by an dry bulb temperature. The resistance 31 is a elongated Y hygroscopic element as disclosed in hygroscopic element. of the type disclosed, for the Allen Patent No. 1,781,153. _ 15 example, in the December, 1939, issue oi’ the Fig. 6 illustrates how inductances may be ad Journal of the National Bureau of Standards, the justed conformably with relative humidity changes and dry bulb temperature» changes for causing resultant effects which vary conform ‘ electrical resistance of which varies conform ably with changes in the relative humidity of the air. The effects of the adjustments of the con ably with wet bulb temperature changes. The 20 denser 36 and the resistance 31 are to change inductance 125 has its turns coated with hygro the frequency of oscillation of the tube 35. A scopic material whereby the distributed capacity decrease in the impedance of the circuit caused by varies conformably with relative humidity a decrease in capacity causes a higher frequency changes, thus variably tuning theinductance. oscillation. A decrease in resistance resulting The inductance 25 is connected to the rotor 25 from an increase in humidity causes a decrease in winding of the variometer 26, which winding may frequency. The impedance changes are so ad be the inductance 21 of Fig. '7, while the oscillator justed that the frequency meter i5 connected is connected by the wires 30 and 3| to two wind across the inductance 3B, may be calibrated to ' ings on the stator of the variometer, which wind read wet bulb temperatures. The contact 39 on ings may be the inductances 28 and 29 of Fig. ’1. 30 the scale of the meter is set at the desired con A coiled lai-metallic member rotates the rotor of trol point so that the indicator 40 closes a con the variometer to vary the eil'ect of the induct trol circuit when a predetermined wet bulb tem ance 25 upon the field from the oscillator, con- A formably with dry bulb temperature changes, for decreasing the coupling conformably with dry bulb temperature increases and for increasing the coupling conformably with dry bulb tempera ture decreases. The over all effect is, as de ` perature occurs. The embodiment of the invention illustrated by Fig. 10 comprises the hygroscopic resistor 5!! which may be of the type disclosed in the F. W. Dumore Patent No. 2,285,421, the resistance of which decreases conformably with humidity in creases in the air to which it is exposed. The scribed in connection with Figs. 1-5,v_that the os~l cillator is modliied coniormably with Wet bulb 40 resistor bil is connected in series with the vari temperature changes. able resistor bi, the contact arm ‘52 of which is 'i discloses an oscillator circuit of the type varied by the bellows 53 connected by the tube disclosed in said Stevents et al. patent and in 5i with the dry bulb thermometer b5. which a vacuum tube oscillator has an output The resistors 5B and bi in series are connected -voltage which falls with increase in load caused in shunt with the D. C. voltage source 58 and by increased power absorption between the elec» with the control grid circuit of the thermionic trodes it and ii, which increased power absorp tube b1. tion is caused by increase in the moisture ab The energizing winding of the relay E8 is con sorbed in the dielectric on the electrodes l0, II, nected to the anode 59 of the tube 51'and to and it. lTi‘he decrease in voltage is indicated by the junction point of the resistor 5I! and one the meter i5. ‘bi-metallic member i2 acts as previously described, to increase the spacing between the electrodes and II, and thereby to decrease the power absorption, upon increase in dry bulb tem- f perature. The meter I5 responds to the differ ences in power absorption due to moisture ab sorption and separation of the electrodes, and may be calibrated to indicate the resultant wet bulb temperatures. The relay I6 connected in series with the meter I5 is adjusted so that it releases its armature 33 to close a control circuit when a, predetermined wet bulb temperature occurs. side of the potentiometer 6I. The anode 59 is also connected 'through the resistor 64 to the con tact arm 62 of the potentiometer 6I, the other side of which is connected to the positive side of the B supply source S3. The minus side of the source 63 is connected to ground. The relay 58 is connected in a bridge circuit in the tube anode circuit and when the poten tiometer 6I is adjusted until the "relay contacts >swing wide open, the bridge circuit is balanced. An increase in voltage at the grid of the tube will cause the bridge circuit to unbalance and the relay contacts to close. Such a voltage in crease will be caused by an increase in wet bulb ~ The oscillator circuit illustrated by Fig. 8 is of 65 temperature, the resistance of the resistor 50 the type disclosed in said Termen patent and has a vacuum tube oscillator 3€, the anode current of which increases conformably with the power decreasing conformably with humidity increases and the effective resistance of the resistor El in creasing conformably with dry bulb temperature absorption caused by moisture absorbed by the dielectric between the condenser electrodes I0 70 increases. The constants of the circuit are so selected that the bridge circuit is unbalanced to and II. The bi-metallic element I2 acts to de crease the effect of this loss upon increases in dry bulb temperature by increasing the spacing be tween the electrodes. The meter I5 which in thisl case, preferably is a milliammeter, may thus close the relay contacts when a predetermined wet bulb temperature occurs. ‘ The embodiment of the invention illustrated by Fig. 11 is similar to that of Fig. 10 except that lno bridge circuit is included in the anode circuit 2,412,782 of the thermionic control tube, the circuit being balanced by auxiliary variable resistors in the anode and grid circuits. The common elements of Figs. 10 and 11 have been given the same ref erence characters. and which may be calibrated in terms of wet bulb temperatures. Fig. 15 illustrates a control circuit similar to that of Fig. 11 except that no hygroscopic resistor is used. The resistor 9d is oi’ a metal such as platinum or- nickel, the resistance of which in creases with increase in temperature. The re sistor 9_5 in series therewith has its contactarm adjusted by a coiled hygroscopic element 9B such 85 and the D. C. voltage source 55 are connected in series across the grid circuit of the tube 55. 10 as that illustrated by Fig. 4 so that increases in humidity cause decreases in the effective resist The variable resistor 66 is connected in series ance of the resistor 95. The sum: of the resist with the anode 59, the energizing winding of the ances of the two resistors determines the po relay' 55 and the Bl supply 63. The resistors 55 tential on the grid of the tube 5l as described in -and 86 are adjusted to provide that the contacts connection with Fig. 11, the tube passing suin of the relay 58 close at a predetermined wet bulb cient anode current to actuate the relay 58 at a temperature. selected wet bulb temperature. The tube 51 of Figs. 10 and 11 may be of the Fig. 16 illustrates a control circuit in which no so-called high vacuum type, or maybe of the vacuum tube is necessary. The A. C, source |00 -gaseous type in which a ñash over occurs at a may be 60 cycle alternating current. It is con predetermined grid potential. nected by the wire iûi to the resistor |52, the The embodiment of the invention illustrated by Fig. l2 uses a cold cathode tube, the cathode _ contact arm |03 of which is moved by the bellows |04 of the dry bulb thermometer |05. The hy 69 of which is connected to one side of the volt groscopic resistor |06 is connected by the contact age source 69 which may be A. C. or D. C. The arm |03 and to one side-of the input of the full other side of the supply 69 is connected to the Wave, copper oxide rectifier lû'l. The other side hygroscopic resistor 50 which is in series with of the input of the rectiñer is connected by the the variable resistor 5|, the balancing resistor wire |08 to the other side of the source |00. The 15 and the starting anode 1| of the tube El. The output of the rectifier is connected to the ohm energizing winding of the relay 58 is connected meter or voltmeter |09. 30 in series with the said other side of the source The resistance of the hygroscopic resistor |06 59 and with the anode 'i2 of the tube 6T. With decreases with increase in humidity while the re .such a cold cathode tube, anode current iiows sistance of the resistor |02 increases with increase when the starting the anode is raised to the igni in dry bulb temperature. The constants of the tion potential. The constants of the circuit may circuit can be so chosen and the meter |09 so cali be chosen so that when a predetermined wet bulb brated that it reads wet bulb temperatures, and its temperature occurs, the sum of the resistances pointer Ilil closes a circuit at a predetermined of the resistors 50 and 5| decreases to permit wet bulb temperature to actuate a relay. the starting anode 7| to be charged from the The impedance of an electric circuit is made source 69, to the ignition potential. up of the resistance and of the capacitive and in 40 The oscillator circuit of Fig. 13 includes a re ductive reactance of the components of the circuit. sistance tuned oscillator similar to that of Fig. 9, Varying one or more of these as described in the except that instead of the dry bulb lthermostatic foregoing, varies the electrical impedance. element varying a condenser, it varies a resistor In the annexed claims, “conformably” means in series with the hygroscopic resistor. The re in proportion. For example, a humidity increase sistance of the hygroscopic resistor 3l decreases from minimum to maximum may cause a resist with increase in humidity. The resistor 85 in ance to decrease from maximum to minimum, and series with the resistor 8| has its resistance in a dry bulb temperature increase from minimum creased with increases in dry bulb temperature. by to maximum may cause a resistance to increase the expansion of fluid from the thermometer 83 from minimum to maximum. into the bellows 82 which shifts the contact arm The devices referred to in the annexed claims Bi of the resistor 86. Increases in resistance may be thermostats for actuating controls at se cause increases in frequency. The constants of lected Wet bulb temperatures, or may be thermom the circuit are so adjusted that the frequency eters for indicating wet bulb temperatures, or meter i5 may be calibrated to read Wet bulb tem peratures and to actuate controls at a predeter- ' ‘ may be combined thermostats and thermometers. ' While embodiments of the invention have been mined wet bulb temperature. described for the purpose of illustration, it should The oscillator circuit of Fig. 14 is a conven be understood that the invention is not limited tional, relaxation type oscillator. The hygro to the exact apparatus, arrangements of ap scopic resistor 85 and the resistor 86, the resist paratus and circuits illustrated, as modiñcations ance of which is-adjusted by the bellows Si of thereof may be suggested by those skilled in the .the thermometer 88, are connected in series in art without departure from the essence of the the control grid circuit of the thermionic tube invention. ' B9. What is claimed is: The resistors 9@ and Si are limiting resistors l. A device responsive to changes in the wet and the condenser 9| is the feedback condenser. bulb temperature of air, comprising an electric The resistance of the resistor 35 decreases with circuit, means in said circuit responsive to humid humidity increases and the resistance of the re ity increases in the air for varying the electrical sistor 86 increases with dry bulb temperature in impedance of said circuit conformably with creases. The sum of the resistances of the re changes in the humidity of the air, and means in sistors 85 and 55 determines the frequency of said circuit responsive to dry bulb Itemperature osclllation'of the tube 89 and is an indication increases in the air for oppositely varying the of the wet bulb temperature. The output of the impedance of said circuit conformably with -tube $5 may beampliñed by the audio ampliñer changes in the dry bulb temperature of the air. '92 to which the frequency meter 93 is connected 2. A device responsive to changes in the wet With reference to Fig. ‘11, the hygroscopic re sistor 50, the variable resistor 5i adjusted by the thermometer 55, the auxiliary variable resistor 2,412,782 ’ bulb temperature of air, lcomprising an electric circuit, means in said circuit responsive to humid ity changes in the air for decreasing the electri 10' changes in the air for cDpOsitely changing the strength of said current conformably with in creases in the dry bulb temperature of the air. cal impedance of said circuit coniormably with 8. A device responsive to changes in the wet increases in the humidity of the air, and means Cn bulb temperatureof air, comprising means in in said circuit responsive to dry bulb temperature cluding hygroscopic means exposed to the air for changes in the air for increasing the impedance producing an electric current iiow, the strength thereof conformably with increases in the dry of which varies in one direction conformably with bulb temperature of the air. increases in the moisture content ‘of the air, and 3. A device responsive to changes in the wet 10 means including means responsive to dry bulb bulb temperature of air, comprising an electric temperature changes in the air for varying the circuit, means providing an electric capacity in strength of said current now in the opposite di said circuit, means including means responsive rection conformably with increases in the dry to humidity changes in the air for varying the bulb temperature oi the air. , capacitive reactance of said capacity conform 1.5 9. A device responsive to changes in the wet ably with increases in the humidity of the air, bulb temperature of air, comprising an electric and means including means responsive to dry bulb temperature changes in the air for oppositely condenser, means including means responsive to humidity changes in the air for increasing the varying the capacitive reactance of said capacity capacity of said condenser conformably with in conformably with increases in the dry bulb tem 20 creases in themoisture content of the air, and perature of the air. means exposed to the air for decreasing the ca 4. A device responsive to changes in the wet pacity of said condenser coni’ormably with in bulb temperature of air, comprising an electric creases in the dry bulb temperature of the air. circuit, means in said circuit responsive to hu 10. A device responsive to changes in the wet midity changes in the air for varying the resist 25 bulb temperature of air, comprising means for ance of said circuit conformably with increases in producing an alternating electric current, means the humidity of the air, and means in said cir including hygroscopic means exposed to the air cuit responsive to dry bulb temperature changes for varying the frequency of said current con in the air for oppositely varying the resistance formably with moisture increases in the air, and of said circuit conformably with increases in the 30 means including means responsive to dry bulb dry bulb temperature of the air. temperature changes of the air for oppositely 5. A device responsive to'changes in the wet varying the frequency of said current conform bulb temperature of air, comprising an electric ably with increases in the dry bulb temperature circuit, a resistor in said circuit, means respon of the air'. sive to humidity changes in the air for decreas 35 11. A device responsive to changes in the wet ing the resistance of said resistor conformably bulb temperature oi' air, comprising means for with increases in the humidity of the air, a sec ' producing an alternating electric current, means , ond resistor in said circuit, and means responsive including hygroscopic means exposed to the air to dry bulb temperature changes in the air for for decreasing the frequency of said current con increasing the resistance of said second resistor 40 formably with increases in the moisture content conformably with increases in the dry bulb tem of the air, and means including means responsive perature of the air. to dry bulb temperature changes of the air for 6. A device responsive to changes in the wet increasing the frequency of said current con bulb temperature of air, comprising an electric formably with increases in the dry bulb tempera circuit. an electric resistor in said circuit exposed 45 ture of the air. to the air and having a resistance which increases 12. A device responsive to changes in the wet conformably with increases in the dry bulb teln-bulb temperature of air, comprising an electric perature of the air, and means in said circuit re circuit. a pair of resistors in series in said circuit. sponsive to humidity changes in the air for de means responsive to changes in the humidity of creasing the resistance of said circuit conform 50 the air for decreasing the resistance of one of ably with increases in the humidity of the air. said resistors conformably with increases in the '1. A device responsive to changes in the wet humidity of the air. and means responsive to bulb temperature of air, comprising means in changes inthe dry bulb temperature of the air for cluding means exposed to air, providing an elec increasing the resistance of the other of said re tric current flow, means for changing the strength 55 sistors conformabiy with increases in the dry of said current conformably with increases in the _ bulb temperature of the air. moisture content of the air, and means including means responsive to dry bulb temperature ROBERT T.' PALMER.